Transparent image-recording elements are primarily intended for viewing by transmitted light, for example, observing a projected image from an overhead projector. In a typical application, the viewable image is obtained by applying liquid ink dots to an ink-receptive layer using equipment such as ink jet printers involving either monochrome or multicolor recording.
It is known that the ink-receptive layers in transparent image-recording elements must meet stringent requirements including, an ability to be readily wetted so there is no "puddling", i.e., coalescence of adjacent ink dots that leads to non-uniform densities; an earlier placed dot should be held in place in the layer without "bleeding" into overlapping and latter placed dots; the layer should exhibit the ability to absorb high concentrations of ink so that the applied liquid ink does not run, i.e., there is no "ink run off"; a short ink-drying time, and a minimum of haze. To meet these requirements, the ink-receptive layers of the prior art have been prepared from a wide variety of materials. One class of materials that has been described for use in ink-receptive layers of transparent image-recording elements is the class of vinyl pyrrolidone polymers. Typical patents are as follows:
U.S. Pat. No, 4,741,969, issued May 3, 1988, describes a transparent image-recording element having an ink-receptive layer formed from a mixture of a photopolymerizable, double-bonded aniomic synthetic resin and another polymer such as a homo- or copolymer of N-vinyl pyrrolidone. The mixture is cured to provide the ink-receptive layer.
U.S. Pat. No. 4,503,111, issued Mar. 5, 1985, describes a transparent image-recording element for use in ink jet recording and having an ink-receptive layer comprising a mixture of polyvinyl pyrrolidone and a compatible matrix-forming hydrophilic polymer such as gelatin or polyvinyl alcohol.
Unfortunately, transparent image-recording elements that have been described in the prior art and employ vinyl pyrrolidone polymers in ink-receptive layers have generally failed to meet the stringent requirements needed to provide a high quality image and this has significantly restricted their use.
In addition to the requirements already discussed, an important feature of a projection viewable image is the size and nature of the ink dots that form it. In general, a larger dot size (consistent with the image resolution required for a given system) provides higher image density and a more saturated color image and improves projection quality. A known method of increasing dot size involves applying liquid ink dots to a transparent image-receiving sheet, for example, HP PaintJet Film.TM. (commercially available from Hewlett Packard Company, Palo Alto, Calif. ) using an ink jet printer. The sheet is dried for a short time, for example, 5 minutes, and inserted into a transparent plastic sleeve which protects the sheet and controls development of the dots. The sleeve compresses the dots and their size is increased to provide greater image density and color saturation upon projection of the image. Although this method is effective, it would be desirable to achieve appropriate dot size without the inconvenience of handling a separate sleeve.
In recently issued U.S. Pat. No. 4,903,039, issued Feb. 20, 1990, there is disclosed a transparent image-recording element adapted for use in a printing process in which liquid ink dots are applied to an ink-receptive layer such as an ink jet printing process where liquid ink dots are applied to an ink-receptive layer that contains a vinyl pyrrolidone polymer and particles of a polyester, poly(cyclohexylenedimethylene-co-oxydiethylene isophthalate-co-sodio-sulfobenzenedicarboxylate), dispersed in the vinyl pyrrolidone to control ink dot size and to provide a high quality projection viewable image. The result is achieved in a simple and expedient manner by varying the concentration of the polyester in the layer as described therein. Such elements constitute a significant advancement in the art by providing transparent image-recording elements which are adapted for use in printing processes where liquid ink dots are applied to an ink-receptive layer in which the ink dot size can be easily controlled. A disadvantage exists, however, with respect to these elements in that the surfaces of the ink-receptive layers on which the liquid ink dots are applied exhibit, after drying, a coarse or roughened texture much like that of very fine sandpaper, so that the surfaces are not smooth or silken to the touch. Although this might not appear at first impression to constitute very much of a problem, it constitutes quite a major problem with respect to potential customer acceptance in that many people who purchase and or work with transparent image-recording elements prefer, if not insist upon, transparent image-recording elements in which the ink-receiving surfaces are smooth or satiny to the touch.
Thus, it would be highly desirable to be able to provide a transparent image-recording element adapted for use in a printing process in which liquid ink dots are applied to an ink-receptive layer, such as an ink jet printing process, which not only possesses all of the benefits and advantages of the transparent image-recording elements disclosed and described in the aforementioned U.S. Pat. No. 4,903,039, including the ability of the ink-receptive layer to control ink dot size and to provide high quality projection viewable images but, in addition, one in which the ink-receptive layer exhibits an enhanced or improved smoothness.
The present invention provides such a transparent image-recording element. The invention also provides a printing process in which liquid ink dots are applied to the ink-receptive layer of the aforementioned element.